Method for determining iso-paraffins in hydrocarbon mixtures



Patented Apr. 15, 1947 rric METHQD FUR DETERMINENG ESQ-PARA FINS 1N HYDROCARBGN MEXTURES No Brawing. Application February 4:, 1943, Serial No. 474,748

19 Claims. 1

This invention relates to a method for determining the amount of branched chain hydrocarbons in a hydrocarbon mixture and is more particularly concerned with a chemical method of determining the content of hydrocarbons having a carbon atom to which is linked a single hydrogen atom, in a hydrocarbon mixture.

The wide-spread use of isomerization processes for converting straight chain paraffins to iso-paraifins and the alkylation of iso-paraffins by means of olefins to form iso-octane have necessitated development of methods for determining the iso-hydrocarbon content of the resulting products.

The only chemical method in general use for determining the iso-hydrocarbon or the tertiary hydrocarbon content of a hydrocarbon mixture is the Moldavskii method, which involves chlorination with antimony pentachloride-chloroform solution in the presence of phosphorus pentoxide followed by titration of an aliquot portion of the sample with potassium bromate solution using methyl orange as indicator.

We have tried this method using known tures of n-heptane and 2,2,4-trimethylpentane but the results obtained were so unreliable that one mixture could not be distinguished from another with certainty. In many cases there was almost as much reaction with normal saturated hydrocarbons as with saturated iso-hydrocarbons. The titration differences are often so small as to amount to only a few tenths of a cubic centimeter of titrating solution, a difference too small to be a reliable indication of the difference in composition of the mixtures. Moreover, in many cases it is impossible to get check results on different samples of the same mixture. The presence of olefins and/or aromatics inter" feres with the Moldavskii test and frequently the color change of the indicator during titration is fleeting so that the end point is doubtful. in somecases a color change to red occurs before the hydrocarbon mixture is titrated.

Our invention obviates the aforementioned defects in the Moldavskii method and enables one to make determinations which digress from less than 1% to a maximum of approximately 5% from the actual percentage iso-hydrocarbon content of the mixture.

One of the objects of our invention is to provide an improved method for determining the saturated iso-hydrocarbon content of hydrocarbon mixtures.

Another object of our invention is to provide a method for catalyzing the chlorination of saturated isohydrocarbons.

A further object of our invention is to provide an improved treating agent for use in determining the saturated iso-hydrocarbo-n content of hydrocarbon mixtures.

A still further object of the invention is to provide a qualitative test for the presence ofunsaturated and aromatic hydrocarbons in hydrocarbon mixtures.

Other objects of the invention will become apparent from the following description.

In carrying out our method it is important to remove unsaturated and aromatic hydrocarbons from the mixture as the first step in the event they are present. The presence of unsaturated and aromatic hydrocarbons can be determined qualitatively by adding to a portion of the hydrocarbon mixture, a solution of antimony pentachlo-ride in dry chloroform and noting whether any color change occurs. If an immediate change in color (from a darkening of the color of the reagent up to a brownish red color) takes place the presence of unsaturated hydrocarbons or aromatic hydrocarbons is indicated. The re action between aromatics or unsaturates and antimony pentachloride takes place rapidly, whereas several hours are required for appreciable reaction of antimony pentachloride in chloroform solution with saturated hydrocarbons at ordinary temperature.

In the event it is necessary to remove unsaturated and/or aromatic hydrocarbons, this may be accomplished by treating the hydrocarbons with 109% sulfuric acid in two treatments at room temperature or, better, at a temperature of 32-40" F., followed by three successive treatments with 98% sulfuric acid at approximately the same temperature. In each treatment the volume of acid to hydrocarbon mixture used is preferably 1 to 2 parts of the acid to 1 part of the hydrocarbon mixture. The treated mixture is neutralized with caustic soda and washed with water. Some variance from the above directions may be permitted. For example the second treatment with sulfuric acid may be made with 98% sulfuric acid, this usually necessitating additional treatment with 98% sulfuric acid. The ratio of hydrocarbon to acid may also be adjusted to suit the particular mixture being treated, larger amounts of acid being required with a higher content of unsaturated and/or aromatic hydrocarbons.

In the event that the hydrocarbon mixture contains halogen or nitrogen compounds it is necessary to fractionally distill oil the hydrocarbons, leaving the higher boiling halogen or nitrogen compounds as the residue. Certain chloro and nitro-compo-unds (e. g. those containing a carbon atom attached to a single hydrogen atom) interfere with the determination since they react slowly with the antimony pentachloride used in the determination.

The hydrocarbon sample, free of unsaturated and aromatic hydrocarbons and free of haloand nitro-compounds, is added to an approximately normal antimony pentachloride solution containing 149.5 grams of antimony pentachloride in sufiicient dry chloroform to make the solution up to one liter. One-tenth of 1 cc. of the hydrocarbon mixture is added to 25 cc. of the antimony pentachloride-chloroform solution and thoroughly mixed. Three cc. of the mixture is pipetted into a dry 250 cc. glass stoppered flask and about 0.5 gram of phosphorus pentoxide and about 0.05 gram of dried uranyl nitrate are added to the 3 cc. miXture. The mixture is allowed to stand for 6 to 7 hours at 70-90 F., or preferably over night. The mixture after standing the required length of time is shaken with 10 cc. of concentrated hydrochloric acid, 3 cc. of grain alcohol and 60 cc. of distilled water and then titrated with 0.1 N potassium bromate solution in the presence of two drops of methyl orange indicator until the red color just disappears.

Samples of n-heptane and 2,2,4 trimethylpentane must, with the exception of the fractional distillation step, each be treated in precisely the same manner as the unknown hydrocarbon mixture. If the hydrocarbon mixture is treated with acid to remove unsaturates and aromatics, the n-heptane and 2,2,4 trimethylpentane must be likewise treated. After-the n-heptane and 2,2,4 trimethylpentane havebeen treated and titrated in the same manner as the unknown mixture the per cent iso-hydrocarbon content of the unknown mixture is determined from the following formula:

of 3 cc. of grain alcohol, 10 cc. of concentrated hydrochloric acid and 60 cc. of water gives a persistent red color with 2 drops of methyl orange indicator solution (0.1% solution of methyl orange in water). This treatment is carried out to eliminate any excess chlorine that may be present in the antimony pentachloride.

The temperature at which the test is carried out is important. Best results are obtained when the hydrocarbon mixture is allowed to stand with the antimony pentachloride-chloroform reagent at a temperature of 70-90 F. Lower temperatures give less accurate results due to the fact that less antimony pentachloride reacts with the iso-hydrocarbons at lower temperatures.

Although we have found that uranyl nitrate is the best catalyst for promoting the reaction of the antimony pentachloride with the iso-hydrocarbons, we have found a number of other salts that catalyze the reaction among which are mercuric chloride, zinc chloride, anhydrous ferric chloride, zirconium nitrate, thorium nitrate, thallium nitrate, chromic nitrate, cerium nitrate, potassium nitrate, mercurous nitrate, cobalt nitrate, cadmium nitrate, calcium nitrate and cupric nitrate. The presence of the catalyst is extremely important for the reason that the antimony pentachloride reacts with straight chain and cyclo-parafiins as well as with branched chain hydrocarbons, making it difiicult to obtain any large difference in titration between them. The catalyst promotes reaction of the antimony pentachloride reagent with the iso-hydrocarbons without materially increasing the rate of reaction with normal paraffins and cyclo-parafiins not containing a carbon atom attached to a single hydrogen atom, thereby enabling one to obtain large difierences in titration. The difierences which may be obtained by using uranyl nitrate are set forth in the following table giving the results on the titration of a 3 cc. aliquot portion of difierent hydrocarbons prepared by adding one-tenth of a cc. of the hydrocarbon to 25 cc. of antimony pentachloride-chloroform solution,

10 cc. KBrO solution (sample)cc. KBrO solution (nheptane) cc. .KBrO; solution (2,2,4-trimethylpentane)cc. KBrO; solution (n-heptane) In order to eliminate difiiculty experienced with fleeting end point or disappearance of color before titration, it is advisable to prepare the antimony pentachloride-chlo-roform solution in the following manner: Chloroform is shaken twice with equal volumes of concentrated sulfuric acid then washed twice or three times with approximately an equal volume of distilled waterfollowed by washing with an equal volume of 5% aqueous sodium carbonate and then again washed three times with an equal volume of water after which it is allowed to stand over night in the presence of phosphorus pentoxide. The chloroform is then distilled and again allowed to stand over night over phosphorus pentoxide. This treatment insures that the chloroform is pure and dry. The antimony pentachloride is then dissolved in the chloroform. A portion (approximately cc.) of the approximately normal solution of antimony pentachloride in the chloroform prepared as just described is then shaken with approximately 10 grams of powdered antimony and then decanted from the antimony and the resulting solution carefully added to the portion of the antimony pentachloride solution not shaken with antimony until a small portion of the resulting mixture when shaken in the presence prepared as previously described, adding to a 3 cc. aliquot portion thereof 0.5 gram of phosphorus pentoxide and .05 gram of uranyl nitrate, allowing it to stand over night, shaking it with 10 cc. of concentrated hydrochloric acid, 3 cc. grain alcohol and 60 cc. of distilled water, after which it was titrated with 0.1 N potassium bromate solution with two drops of methyl orange indicator until the red color just disappears.

Table 11 gives the results obtained on three known mixtures using our method of analysis. Accuracy within 5% of the actual value is considered good for this type of determination.

Table II Per cent Isopara'flin' m mixture Per'cent Isoparaifin found byanalysis Composition of mixture (per cent by volume) Our method will determine the combined amount of those saturated hydrocarbons containing a carbon atom in combination with asingle hydrogen atom including iso-paraffins such as isobutane and iso-octane and naphthenic laydrocarbons such as methylcycloh'exane, ethylcyclohexan and decalin. The determination is not accurate for such compounds as neohexane and neopentane which are generally classed as isoparairlns but which do not contain a carbon atom in combination with a single hydrogen atom. The determinations on the latter type of compounds are generally low.

It will be seen, therefore, that we have discovered a method for catalyzing the reaction of antimony pentachloride with hydrocarbons containing tertiary carbon atoms and by the use of such catalysts have improved the accuracy of isohydrocarbon determinations. It is also apparent that we have discovered other defects in the present chemical method for determining iso-hydrocarbons in hydrocarbon mixtures and have found means for curing these defects.

We claim:

1. In the method of determining the amount of hydrocarbons having a. carbon atom with a single hydrogen atom linked thereto in a hydrocarbon mixture involving reaction of the mixture with antimony pentachloride followed by titration with alkali metal bromate solution, the step of reacting the antimony pentachloride with the hydrocarbon mixture in the presence of a uranyl salt capable of catalyzing the reaction of the antimony pentachloride with said hydrocarbons at a temperature below that at which substantial chlorination of normal paraffin hydrocarbons occurs.

2. Method in accordance with claim 1 in which the uranyl salt is uranyl nitrate.

3. In a method for determining the content of hydrocarbons having a carbon atom with a single hydrogen atom linked thereto in a hydrocarbon mixture involving reaction of the mixture with antimony pentachloride followed by titration with alkali metal bromate solution the steps of reacting the hydrocarbon mixture with a solution of antimony pentachloride in dry chloroform in the presence of phosphorus pentoxide and a uranyl salt capable of catalyzing the reaction between said hydrocarbons and antimony pentachloride at a temperature below that at which substantial chlorination of normal paranin hydrocarbons occurs.

4. Method in accordance with claim 3 in which the uranyl salt is uranyl nitrate.

5. Method in accordance with claim 1 including the step of removing the unsaturates and aromatics contained in said mixture prior to reaction thereof with antimony pentachloride.

6. In a method for determining the amount of hydrocarbons having a carbon atom with a single hydrogen atom linked thereto, in a mixture containing straight-chain saturated hydrocarbons, involving selective chlorination of the first mentioned hydrocarbons with antimony pentachloride followed by titration with alkali metal bromate solution, the step of removing from said mixture prior to reaction thereof with antimony pentachloride, chloroand nitro-compounds contained therein.

7. A method for determining the amount of hydrocarbons having a carbon atom with a single hydrogen atom linked thereto, in a, mixture of hydrocarbons which comprises reacting said mixture with a solution of antimony pentachloride in dry chloroform in the presence of phosphorus pentoxide and a uranyl salt capable of catalyzing the reaction between said hydrocarbons and antimony pentachloride, at a temperature of about 70 to F. and titratin the reaction product with alkali metal bromate solution in the presence of methyl orange indicator.

8. A method in accordance with claim 7 in which the unsaturates and aromatics are removed from the mixture prior to reaction with the antimony pentachloride.

9. A method in accordance with claim 7 in which the mixture is fractionally distilled to remove chloroand nitro-hydrocarbons prior to reaction with antimony pentachloride.

10. The method of determining the amount of hydrocarbons having a carbon atom linked to a single hydrogen atom, in a mixture of hydrocarbons which comprises removing from said mixture unsaturates, aromatics, nitro compounds and chloro compounds, adding the resulting mixture to a normal solution of antimony pentachloride in dry chloroform in the ratio of 0.1 cc. of mixture to 25 cc. of chloroform solution, adding approximately 0.5 gram of phosphorus pentoxide and approximately 0.05 gram of dry uranyl nitrate for each 25 cc. of solution, allowing the mixture to stand for at least 6 hours in a covered vessel at a temperature of 70-90 Fl, mixing a small portion of the reaction liquid with concentrated hydrochloric acid, grain alcohol and water in the ratio of 3 cc. of reaction liquid, 10 cc. of hydrochloric acid, 3 cc. of grain alcohol and 60 cc, of water, titrating the resulting solution with 0.1 normal potassium bromate solution in the presence of methyl orange indicator until the red color just disappears, and determining the quantity of said hydrocarbons present from the aforesaid titration and similar titrations of normal heptane and 2,2,4-trimethylpentane treated in exactly the same manner as the hydrocarbon mixture.

11. Method in accordance with claim 10 in which the unsaturates and aromatics are removed by treating the hydrocarbon mixture with concentrated sulfuric acid.

12. Method in accordance with claim 10 in which the chloroand nitro-compounds are removed by fractional distillation.

13. Method in accordance with claim 10 in which the unsaturates and aromatics are removed by treating the hydrocarbon mixture with sulfuric acid and the chloroand nitrocompounds are removed by fractionally distilling the hydrocarbon mixture.

14. The method of chlorinating saturated hydrocarbons containing a tertiary carbon atom which comprises contacting a mixture of said hy- 7 drocarbons and antimony pentachloride in a. dry state with a uranyl salt at a temperature of 70-90 F.

15. Method in accordance with claim 14 in which the uranyl salt is uranyl nitrate.

16. The method of chlorinating saturated hydrocarbons containing a tertiary carbon atom in a mixture containing straight chain saturated hy drocarbons, without substantially chlorinating said straight chain saturated hydrocarbons comprising reacting said mixture of hydrocarbons with antimony pentachloride in a dry state in the presence of a, small amount of 3. uranyl salt at a temperature between TO-90 F.

17. Method in accordance with claim which the uranyl salt is uranyl nitrate.

18. Method in accordance with claim 16 in which the antimony pentachloride is dissolved in dry chloroform and the resulting solution reacted with the hydrocarbon mixture.

19. The method of chlorinating saturated hydrocarbons containing a tertiary carbon atom in a mixture containing straight chain saturated hydrocarbons, without substantial chlorination of of time sufiicient to chlorinate said first mentioned hydrocarbons, but insufficient to chlorinate said straight-chain saturated hydrocarbons.

GEORGE W. AYERS. ERSKINE E. HARTON, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,677,831 Krause July 17; 1928 1,754,656 Strosacker Apr. 15, 1930 OTHER REFERENCES Isomerization of Pure Hydrocarbons, Eglofi et al., Reinhold Pub. Corp. (1942) pages 407-419. (Copy in Div. 31.) 

